Corrosion of magnesium

Magnesium alloys have relatively poor corrosion resistance which has been one of the reasons for lack of the widespread application of these alloys. One of the major challenges in the use of Mg alloys is their high susceptibility to corrosion attack [33]. There are three main factors that contribute to the poor corrosion resistance of magnesium alloys:

a) Magnesium is the most active metal used in engineering applications [34].

Thermodynamically, magnesium should react completely with oxygen and with water [35].

b) Susceptibility to internal galvanic attack caused by alloying or impurity elements and conditions which impede the stability of the protective film [36].

c) The oxide/hydroxide passive film on magnesium is much less stable than passive films formed on other metals such as iron or aluminium. This film has poor pitting resistance [37].

In a corrosive environment, pitting or other forms of local corrosion occurs as a result of film breakdown [35]. So magnesium is less resistant to acidic or saline environments. Since the corrosion resistance of magnesium alloys depends both on the concentration of critical chloride ions and the pH of the medium. So the corrosion rate increases for example with the concentration of chloride ions for any value of pH [38]. Sulphates, phosphates and nitrates attack magnesium but not to the same extent as chlorides [4]. Corrosion of magnesium alloys in the presence of chloride ions usually starts with the formation of irregular pits extendendly occupying the entire surface [4]. However this mechanism is different from the autocatalytic pitting observed in stainless steels, where there is a tendency to the formation of deep pits [39, 40]. This effect is not observed in the magnesium base materials probably due to the increase of pH caused by the formation of hydroxide layer on the surface of magnesium alloys [39, 41-43]. Generally, the corrosion rate is not significant in alkaline media at pH values above 10.5, fulfilling thermodynamics as shown in the Pourbaix diagram [25, 44].

2.2.1 Electrochemical basis of magnesium corrosion

The electrochemical reaction of magnesium in aqueous environments produces magnesium hydroxide and hydrogen gas. Consequently magnesium corrosion is relatively insensitive to the oxygen concentration, although the oxygen concentration is an important factor in atmospheric corrosion [4]. The corrosion attack in aqueous environments often involves micro-galvanic coupling between cathodic and anodic areas. The overall reaction for the corrosion of magnesium could be expressed as follow:

ܯ݃ ൅ ʹܪ_ଶܱ ՜ ܯ݃ሺܱܪሻ_ଶ൅ ܪ_ଶ ^{( 1 )}

This total reaction could be divided into two partial reactions according to reactions (2) and (3).

The anodic partial reaction of Mg dissolution, reaction (2) may involve intermediate steps which produce the monovalent magnesium ion (Mg⁺), with short lifetime [2]. The reduction process of hydrogen ions, reaction (3), and the hydrogen overvoltage of the cathodic phase play an important role in the corrosion of Mg. Low overvoltage cathodes such as Fe, Ni, Co, or Cu facilitate hydrogen evolution, causing a substantial corrosion rate [45]. Furthermore Mg(OH)₂ can form if the solubility limit is exceeds. The reaction product formation is described in reaction (4) [4].

ʹܪ^ା൅ ʹ݁^ି ՜ ܪ_ଶ ሺܿܽݐ݄݋݀݅ܿ ݌ܽݎݐ݈݅ܽ ݎ݁ܽܿݐ݅݋݊ሻ ^{( 2 )} ʹܯ݃ ՜ ʹܯ݃^ା൅ ʹ݁^ି ሺܽ݊݋݀݅ܿ ݌ܽݎݐ݈݅ܽ ݎ݁ܽܿݐ݅݋݊ሻ ^{( 3 )} ܯ݃^ଶା൅ ʹܱܪ^ି՜ ܯ݃ሺܱܪሻ_ଶ ሺܿ݋ݎݎ݋ݏ݅݋݊ ݌ݎ݋݀ݑܿݐ ݂݋ݎ݉ܽݐ݅݋݊ሻ ^{( 4 )}

The corrosion potential of Mg is approximately -2.37 V _NHE^1[46] in aqueous solutions at 25°C. Mg forms magnesium hydroxide film, which can provide some protection over a wide pH range.

Assuming that the protective film on Mg is Mg(OH)₂, the thermodynamics that govern the formation of this film are described by the Pourbaix diagram (Figure 2.1), which shows that Mg²⁺ is stable in most aqueous solutions up to ~ pH=10, above which Mg(OH)₂ is stable.

1 The standard electrode is the normal hydrogen electrode (NHE) or standard hydrogen electrode (SHE) realized by bubbling hydrogen gas over a platinum surface, which has all components at unit activity. The reaction is 2H⁺[1N] + 2e^- ĺ H₂[1 atm]. Potentials are often measured and quoted with respect to reference electrodes other than the NHE.

Figure 2.1 Pourbaix diagram for the Mg-H2O system at 25 °C [25] .

In Figure 2.1 the region of water stability lies between the dashed lines marked a) and b). The different regions are separated by the following reactions:

(1) Mg + 2H2O ĺ Mg(OH)2+H2

(2) Mg²⁺ + H₂O ĺ MgO + 2H⁺ and (3) Mg ĺMg²⁺+ 2e

-The magnesium peroxide (MgO2) is marked in the Figure 2.1 as a guide but it was not taken into account in establish the equilibrium diagram because MgO₂ has not been prepared in the pure state. To obtained this is only by the action of hydrogen peroxide (H2O2) on Mg, on MgO or on the Mg(OH)₂. The lines marked as 10⁰, 10^-2, 10^-4, and 10^-6 represent the activity^{2 [47]} of the species. [4].

Several studies of magnesium suggested that the corrosion of magnesium and magnesium alloys initiates due to localized corrosion, and sometimes the localized corrosion is shallow and extended. Nevertheless the corrosion morphology of magnesium and its alloys depends on the alloy composition and the environmental exposure.

Table 2.1 details the different types of corrosion that occur in magnesium and magnesium alloys [3].

2 Activity (ai) is the effective concentration that takes into account the deviation from ideal behaviour, with the activity of an ideal solution equal to one. The activity value is affected by the concentration, temperature and pressure and normally is determined using an activity coefficient (J_i) to convert from the solute’s mole fraction xi (as a unit concentration) to activity ai using the following formula: a_i=J_ix_i. For ideal solutions, pure and solid substances a_i=x_i thus J_i=1.

Table 2.1 Types of corrosion on Mg alloys and their features [3]

Corrosion type Special features

Galvanic corrosion

ƒ Localized corrosion of the magnesium adjacent to the cathode.

ƒ External cathodes, as other metals in contact with magnesium (galvanic corrosion external).

ƒ Internal cathodes, as second or impurity phases (galvanic corrosion internal).

ƒ Highly susceptible to impurities such as Ni, Fe, Cu.

ƒ The galvanic corrosion rate is increased by: highly conductive medium, large potential difference between anode and cathode, low polarisability of anode and cathode, large area ratio between cathode to anode, and short distance from anode to cathode [44].

Intergranular corrosion

ƒ Slightly susceptible.

ƒ Corrosion is normally concentrated in the area adjoining the grain boundary until eventually the grain may be undercut [2, 30].

Localized corrosion

ƒ Highly susceptible when exposed to chloride ions and in a non-oxidizing medium [48].

ƒ Typically occur as pitting in neutral or alkaline salt solutions [49].

ƒ Heavy metal impurities promote general pitting attack [50].

ƒ In Mg-Al alloys: the pits form by selective attack around the cathodic areas [51].

ƒ Filiform corrosion due to an active corrosion cell moving across the alloys surface, where the head is the anode and the tail the cathode [52].

Stress corrosion cracking (SCC)

ƒ In Mg alloys SCC is mainly transgranular however intergranular SCC occurs when Mg17Al12 precipitates along the grain boundaries (Mg-Al-Zn alloys) [2].

ƒ Alloying elements such as Al or Zn promote SCC [53], but additions of Zr protect against SCC [54].

ƒ Mg is resistant to SCC in alkaline media above pH 10.2, fluoride solutions and neutral solutions containing chlorides [44].

Corrosion fatigue ƒ Cracks propagate in a mixed transgranular-intergranular mode [55].

Corrosion at elevated temperatures

ƒ The oxidation rate of Mg is a linear function of the time indicating a non-protective oxide on the magnesium surface [44].

ƒ Alloying elements such as Al and Zn promote a higher oxidation rate [44].

ƒ Ce and La additions show a lower oxidation rate compared to pure Mg [44].